Memento is the real focus of the set, the other moves are just mildly useful

You're using Memento 9 out of 10 times when you come in

Earthquake is really weak but can be useful for 2HKOing Nidoqueen and Drapion

Sucker Punch is good for getting that extra 20% off some threatening sweeper, but like Earthquake is ridiculously weak

Stealth Rock can be used if you know that you're faster than your opponent and have a turn to spare, and it's obviously good to get up

Can really only be used on offensive teams

[ADDITIONAL COMMENTS]

EVs are pretty simple, Speed is the only thing that really needs to be maxed out, and you can maximize Attack to increase Diglett's nonexistent power or maximize HP instead to increase Diglett's nonexistent bulk

Sash ensures that Diglett will almost always get a Memento off, and can maybe let it switch in and Memento slow stuff like Escavalier

Arena Trap obv

You can use Hone Claws to do some amount of damage if you come in on a Choiced Electric move but no one really uses those much anymore besides Rotom-C which is scary

This set pairs best with things that are really scary after one turn of setup but often struggle to get that turn in; think stuff like Linoone, Omastar, Fraxure, etc.

Whimsicott can start that MEMENTO CHAIN and has Taunt to keep Diglett's Sash intact

A good offensive Spiker can be good too, such as Smeargle, Accelgor, and even Qwilfish can use Taunt to do work

[Other Options]

Choice Band to hit marginally harder

Choice Scarf is kinda outclassed by Focus Sash to increase the chances of getting a Memento off

There's no reason to use Diglett other than Memento

[Checks and Counters]

Kind of a difficult Pokemon to counter, because of Arena Trap and the fact that it typically kills itself with Memento

Aerodactyl has a fast Taunt and beats it

Cinccino has multi-hit moves to break Diglett's Focus Sash

Pokemon with phazing moves can stop the impending setup of Diglett's teammates

RU Co-Leader

As you mentioned in the skeleton, Diglett's niche is very, very small, and the vast majority of the time it would be better to use something like Dual Screens uxie instead of diglett, nonthenless, after watching a few of your matches and trying out diglett myself (way back in Gothitelle meta tho). I'd be ok with giving this a set onsite (although some other people might not be). Just be sure to stress that if you dont need arena trap, theres usually a better option out there. If this does get approved, i might suggest adding Sand Attack in the AC over Sucker Punch, if Diglett gets a free turn to do something other than Memento, it can use Sand Attack to lower the accuracy of the opponent, crippling them even further and reducing the risk of set up, plus, Sucker Punch is mainly filler anyway.

EDIT: decided im not going to QC approve this just yet, theres been a bit of controversy over whether diglett deserves a set, once its all cleared up ill either approve or withdraw my approval.

C&C Leader

I feel like something like this can be covered in a letter, we don't want to encourage people to use something like this in regular battles though because of just how precise you need to play it.

In normal battles the conditions have to be just right for this to work, and there are too many things that can go wrong.

PLUS, if Diglett actually did something outside of simply giving the team Memento support then I'd think about giving it an analysis. But really, Memento is a one-time thing, and the opponent can still fight back even afterwards and fight off a Linoone sweep (hard to do but possible). If Diglett was capable of doing something else like sniping a few relevant threats and then going down with Memento I'd be more inclined to approve it.